richtek fast load transient tool - farnelladjust the pulse load duty-cycle / frequency to see the...
TRANSCRIPT
Buck Converter Stability checks
using
Richtek Fast Load Transient Tool
Richtek Field Application Engineering
December, 2016
CONFIDENTIAL
Converter stability check methods (I)
2
+ Good overview of critical loop parameters
- Complicated measurement
- Prone to noise pick-up and non-linear effects
Frequency Domain Open Loop Gain-Phase Analysis
Perbutation signal
amplitude needs
careful adjustment: too
high amplitude causes
non-linear behavior.
Select for CCM
operation
DC/DC Buck converter
CONFIDENTIAL
Converter stability check methods (II)
3
+ Simple measurement
+ Can show various converter response effects
- Needs some skill to interprete the output waveform
Time Domain Fast Load Transient Analysis
Select for ~30%
dynamic load
Select for CCM
operation
Add electrolytic input capacitor
to avoid input ringing
CONFIDENTIAL
Richtek Load Transient Tool (I)
4
The Richtek Load Transient Tool contains a micro controller that switches a MOSFET on and off with
a certain duty-cycle. By means of jumpers, 7 different pulse load resistors can be selected. The tool
includes an adjustable 10Ω power resistor for setting the static load level. This tool can generate fast
load steps (~500nsec rise/fall times), and the pulse load duty-cycle and frequency are adjustable by
means of push buttons. The tool is battery powered, so it can easily be applied to any voltage
regulator output in your system.
CONFIDENTIAL
Richtek Load Transient Tool measurement setup
5
The Load transient tool is intended to be used for testing voltage regulators
(buck, boost, LDO) with output voltage between 1V and 5V and maximum 5A
current rating, but basically it can be used for testing any voltage regulator
output. Just apply the pulse load leads to the converter output, adjust the
static load resistor for CCM (continuous current mode) or ~ 30% of rated load,
select the pulse load resistor for ~ 30% of rated load, measure the pulse
current and the regulator output voltage across the output capacitors. Adjust
the pulse load duty-cycle / frequency to see the full step load response.
CONFIDENTIAL
Richtek Load Transient Tool schematic
6
The above schematic shows the micro controller that drives the MOSFET switch.
The MOSFET gate drive is designed to generate equal switching speeds with
~500nsec rise/fall times. Reducing or removing C2 can increase the switching
speed, but the actual load current transient speed is mostly determined by the
wiring inductance between the tool and the application. Especially when testing
low voltage supplies (< 2V), it may be necessary to use short, thick wires
between the tool and the application to minimize inductance. Low inductance connection
CONFIDENTIAL
Richtek Load Transient Tool
7
What can you do with the fast load transient tool?
Quickly check
converter loop
stability
Check input
supply stability
Check layout
related problems
….and many more: Vout sag & soar, load regulation, slope compensation issues, estimate
converter bandwidth, duty-cycle limits, check LDO, Boost, Flyback …..
CONFIDENTIAL
Current Mode Buck converter control loop formula’s
8
Standard design values:
1.Set FCONTROL for ~ 1/10 of FSWITCHING
2.FZ to be just below FP_LOAD
3.FP to be close to FZ_ESR
(in noisy application with MLCC Cout, FP can be set between 0.5 ~ 1* FSWITCHING)
CONFIDENTIAL
Current Mode Buck converter
9
Too small CCOMP can result
in lower phase margin
Lower Cout increases
control bandwidth
Bigger RCOMP increases
control bandwidth
Too big CP can result in
lower phase margin
Higher switching frequency
normally requires:
• Lower L1
• Higher RCOMP
• Lower CCOMP
• Lower CP
Components that influence Current Mode Buck converter loop stability
Too high control bandwidth can
cause too low phase margin
CONFIDENTIAL
Fast step load 0.5A ~ 1.5A
slew rate 2A/usec
Example of Fast Transient response vs. Phase Margin
10
Phase Margin
≈ 38 dgrs
Phase Margin
≈ 45 dgrs
Gain-Phase plot:
BW ≈ 55 ~ 45kHz
PM 58 ~ 37 dgrs
Phase Margin
≈ 48 dgrs
Phase Margin ≈ 58 dgrs
CONFIDENTIAL
What else can you learn from Fast Transient response?
11
Response time tR is a rough indication
of the control bandwidth:
For most current mode buck converters:
Bandwidth BW ≈ 0.3 / tR
In this example:
tR = 5.3usec: BW ≈ 0.3/5.3usec = 57kHz
tR
Note: Load step rise time must be << than 1/FC.
Choose load step rise time around 500nsec
CONFIDENTIAL
Actual measurement example
12
Actual Gain-Phase
measurement:
Single bump:
PM ≈ 45 dgs
tR = 3.6usec:
BW ≈ 0.3/3.6usec
= 83kHz
Measured BW = 72kHz
Measured PM = 45.8 dgs
CONFIDENTIAL
Source:
http://ds.murata.com
/software/simsurfing/en-us/ Be aware of MLCC capacitor DC bias and AC ripple characteristics:
Output capacitor value is critical for loop stability!
13
Smaller output capacitor
increases BW!
Small MLCC has
considerable
capacitance drop at
higher DC bias
Always use actual capacitance when designing the control loop!
(if DC bias or AC voltage effect is not specified for your capactor type, ask for it)
Rated capacitance is
measured at 0.5Vac, but
can drop considerably at
lower AC voltage
CONFIDENTIAL
Practical example on output capacitor influence (I)
14
Actual capacitance is 11uF * 0.7 = 8uF: total output capacitance 16uF instead of 44uF
Converter Bandwidth will become 44/16 = 2.75 times higher than design value.
Only 11uF at 3.3Vdc
30% drop at 50mVac
800kHz converter designed
for 2x22uF output capacitance
in 3.3V output application.
Circuit was designed for
BW = 69kHz and PM = 59dgs
Designer selects 2x 22µF / 6.3V 0805 MLCC: GRM21BR60J226ME39L
CONFIDENTIAL
Practical example on output capacitor influence (II)
15
Circuit simulation using online
Richtek Designer tool and
using 16uF output capacitance
confirms the step load result:
BW = 156kHz and PM = 26dgs
Step load shows
severe ringing with
ringing frequency
of 139kHz
(Ringing frequency
indicates Fc value)
You can reduce RCOMP to
reduce the BW to original
design value for better PM.
CONFIDENTIAL
ACOTTM Buck converter stability
16
Adding CFF will increase
system damping
Higher Cout reduces
system damping
Higher Vout (> 2.5V)
reduces system damping
ACOT buck converters are much less critical in stability compared to
current mode buck converters.
Normally applications with higher duty-cycle (> 20%) or large value output
capacitors need to add some CFF to increase system damping.
12V input
CONFIDENTIAL
ACOT converter stability: How to tune Cff
17
The feed-forward capacitor plays a role in the
damping of the ACOT control loop, especially at
higher duty-cycle applications like 12V 5V. For
low duty-cycle applications like 12V 1V it is
normally not needed. The value of Cff for a specific
ACOT converter depends on duty-cycle, COUT value,
inductor value and R1 value.
1. Apply a fast step load and if it shows
ringing, measure the ringing frequency
In this 12V 5V example: fRING=59.5kHz
2. Calculate Cff by the formula:
In this example: (R1 = 120k)
Practical method to find Cff value:
Without Cff
After adding Cff = 27pF: well-
damped step response
With Cff
Relevant Richtek application note
AN038: Fast Load Transient Testing
CONFIDENTIAL
Richtek Application AN038 Practical hints and tips on converter stability testing using fast load transients
Remote sense
CONFIDENTIAL
Richtek Designer online tool Full simulation of application circuit